Removal of suspended solids and matter from liquids such as industrial and municipal waste water is accomplished by several methods. Sedimentation is used for heavier than liquid solids, while filtration is used for solid particles of small size, while dissolved air floatation is used for floatable and lighter than liquid suspended matter.
In dissolved air floatation (DAF), air at high pressure is dissolved in a slip stream of the waste water to be treated and introduced into a floatation tank at a low pressure along with the rest of the waste stream. When the pressure is reduced, micrometer size air bubbles are released and rise through the liquid pool contacting and lifting any floatable suspended matter to the surface of the liquid pool. Turbulence and back mixing are caused by numerous actions including mechanical sludge removal means, high transitional flow, and the rising suspended matter and the descending higher density clarified water. These turbulence actions, either alone or in combination create a pervasive critical factor in DAF and greatly reduces the efficiency of floatation units and frequently render them inoperable. The turbulence and back mixing occurs in a gradually ascending zone while the duration of complete travel time of the turbulent zone depends upon the tank height, and amount and size of floatable matter rising with the liberated microbubbles of air. Poor performance of dissolved floatation units is normally contributed to high liquid transfer velocity and improper equipment design. Best performance is achieved when turbulence and back mixing are minimized.
Complete separation of floatable suspended matter fails to occur when the duration for suspended matter rise to the surface exceeds the residence time of the liquid in the floatation tank. In conventional continuous units, there must be a net water movement in order for the water to flow from an inlet to an outlet. A pool velocity approaching zero is a critical objective to achieve maximum theoretical limits. Prior systems have been developed which have attempted to overcome the problems associated with the turbulence and back mixing while also maintaining a net water movement.
In the conventional DAF system floatable matter is mechanically removed from the surface causing further turbulence and mixing. Numerous inventions have been directed to improving the mechanical means for removing the floatable matter while attempting to minimize turbulence. Some of these patents include: Petit et al. U.S. Pat. No. 5,766,484; Chudacek et al. U.S. Pat. No. 5,660,718; Yeh U.S. Pat. No. 5,538,631; Roshanravan U.S. Pat. Nos. 5,437,785; 5,310,485; 5,151,177; Krofta U.S. Pat. No. 4,022,696. These prior inventions recognized the problem of facilitating removal of the floatable layer while also minimizing turbulence caused by the mechanical action of the removal. The prior methods and apparatus include redesigned floatation tanks, improved mechanical devices that gently remove the floated layer, redesigned tank shapes and interior baffles or other means which increase the surface area in the interior of the floatation tank thereby facilitating quicker floatable material rise time.
Although the prior art has been shown to be extremely useful for the stated purposes, they do not provide a system as the present invention which provides an extremely flexible sequential dissolved air floatation system that includes a continuous system flow utilizing any number of floatation tanks, a single fixed inlet/outlet central port for filling and draining each tank, true zero pool velocity, nonmechanical removal of the floated sludge, and a system that is easily expandable and arranged to meet any inlet flow rate need, while the capital and operating cost are low. The present system further includes means for separating and collecting the floated waste material as well as clarified water utilizing a single tank, or multiple sequential tanks. The invention utilizes any number of floatation tanks, each with a conical bottom with a single centrally located port where waste water charger with dissolved air enters the floatation tank and where separated clarified water is removed from the tank followed by floated waste material. The central port is fitted with a series of valves and plumbing which control the flow of waste water charged with dissolved air into the tank, the flow of clarified water from the tank and the removal of floated waste material from the tank. Since the clarified water is removed from the bottom of the tank(s) prior to removal of the separated floated waste, and since no mechanical means are utilized for removing the floated waste, turbulence caused by any floated waste removal is eliminated resulting in true zero pool velocity. Removal of the clarified water from the bottom of the tank(s) allows for removal of clarified water almost immediately after the tank(s) have been filled with waste water charged with dissolved air. In this regard, the system provides numerous options for operation; the floatation tanks may be filled sequentially and drained sequentially to achieve net waste water flow, any number of tanks may be filled in groups sequentially and drained in groups sequentially or as permitted by the speed of floatation in each tank. Additionally, unlike the prior art, the present invention provides for floatation of suspended waste in a pool that has no transitional movement.
The present invention also may include any number of water clarity optic sensors, which monitor the presence of clarified water in the tank at a location near the central port so that an operator may began draining the clarified water from the tank as soon as possible thereby allowing clarified water to be removed while floated waste material rises in the tank. The clarity sensor(s) and valve actuators are electrically coupled to a controller that controls the filling using a liquid level device and draining flow rate of the tank(s) to achieve maximum efficiency.